Conventionally, a vehicle such as station wagon and minivan is formed with a relatively-large opening which allows an occupant to easily get on and out of the vehicle and allows a baggage to be put in and taken out of the vehicle. Such an opening is opened and closed with a sliding door (opening and closing unit) provided with a roller assembly. Since the sliding door is heavy in weight, the vehicle equipped with the sliding door is provided with a sliding door opening and closing mechanism capable of automatically opening and closing the sliding door.
The sliding door opening and closing mechanism is provided with a driving unit which pulls an opening cable and a closing cable so as to move the sliding door in an opening direction and a closing direction. The driving unit has a drum around which the opening cable and the closing cable are wound in respective directions reverse to each other. This drum is rotated in the normal direction or reverse direction so as to pull the opening cable or closing cable, and as a result, it causes the sliding door to move in the opening direction or in closing direction.
Since the driving unit is mounted and disposed in a relatively narrow space in the vehicle, and adapted to drive the heavy sliding door, the driving unit must be small in size and large in output. In order to meet this requirement, a speed reduction mechanism is provided between a driving source (electric motor) of the driving unit and the drum. Such a driving unit provided with a speed reduction mechanism is publicly known as a technique described in for example Japanese Patent Application Laid-Open Publication No. H10-131612 (FIG. 2).
A slide actuator (driving unit) described in Japanese Patent Application Laid-Open Publication No. H10-131612 (FIG. 2) is provided with: a motor (electric motor) serving as a driving source; and a drum having a wire (cable) wound around it. A speed reduction mechanism is provided between the motor and the drum, and with a plurality of gears (spur gears) different in diameter from each other and engaged with each other. Since the speed reduction mechanism is a so-called “spur speed reducer” having a reduction gear ratio which is smaller than that of a worm speed reducer, a low-speed and high-torque motor must be adopted as a motor for this speed reduction mechanism. In many cases, a high-speed motor having a rated rotation number no less than 3000 rpm is adopted as a motor for driving the worm speed reducer.
However, according to the slide actuator described in Japanese Patent Application Laid-Open Publication No. H10-131612 (FIG. 2), since its speed reduction mechanism can be constructed by combination of simple spur gears, it is possible to offer a slide actuator reduced in size (thickness), and reduced in sound noise through adoption of a low-speed motor.